History of life on Earth
See also: Timeline of life on Earth The evolutionary history of life on Earth traces the processes by which living and s evolved, from the earliest emergence of life to the present. Earth formed about 4.5 billion years (Ga) ago and evidence suggests life emerged prior to 3.7 Ga. (Although there is some evidence of life as early as 4.1 to 4.28 Ga, it remains controversial due to the possible non-biological formation of the purported fossils.) The similarities among all known present-day indicate that they have diverged through the process of from a . Approximately 1 trillion species currently live on Earth of which only 1.75–1.8 million have been named and 1.6 million documented in a central database. These currently living species represent less than one percent of all species that have ever lived on earth. The earliest evidence of life comes from and fossils discovered in 3.7 billion-year-old ary rocks from western . In 2015, possible "remains of " were found in 4.1 billion-year-old rocks in Western Australia. In March 2017, putative evidence of possibly the oldest forms of life on Earth was reported in the form of fossilized s discovered in precipitates in the of Quebec, Canada, that may have lived as early as 4.28 billion years ago, not long after the 4.4 billion years ago, and not long after the 4.54 billion years ago. Microbial mats of coexisting and were the dominant form of life in the early Epoch and many of the major steps in early evolution are thought to have taken place in this environment. The evolution of , around 3.5 Ga, eventually led to a buildup of its waste product, , in the atmosphere, leading to the , beginning around 2.4 Ga. The earliest evidence of s (complex s with s) dates from 1.85 Ga, and while they may have been present earlier, their diversification accelerated when they started using oxygen in their . Later, around 1.7 Ga, s began to appear, with s performing specialised functions. , which involves the fusion of male and female reproductive cells ( s) to create a in a process called is, in contrast to , the primary method of reproduction for the vast majority of macroscopic organisms, including almost all (which includes s and s). However the origin and remain a puzzle for biologists though it did evolve from a common ancestor that was a single celled eukaryotic species. , animals with a front and a back, appeared by 555 Ma (million years ago). The earliest complex land plants date back to around 850 Ma, from carbon isotopes in Precambrian rocks, while algae-like multicellular land plants are dated back even to about 1 billion years ago, although evidence suggests that s formed the earliest s, at least 2.7 Ga. Microorganisms are thought to have paved the way for the inception of land plants in the . Land plants were so successful that they are thought to have contributed to the . (The long causal chain implied seems to involve the success of early tree (1) drew down CO2 levels, leading to and lowered sea levels, (2) roots of archeopteris fostered soil development which increased rock weathering, and the subsequent nutrient run-off may have triggered s resulting in s which caused marine-life die-offs. Marine species were the primary victims of the Late Devonian extinction.) appear during the period, while s, along with most other modern originated about during the . During the period, s, including the ancestors of s, dominated the land, but most of this group became extinct in the . During the recovery from this catastrophe, s became the most abundant land vertebrates; one archosaur group, the s, dominated the and periods. After the killed off the non-avian dinosaurs, mammals . Such s may have accelerated evolution by providing opportunities for new groups of organisms to diversify. Earliest history of Earth | from=- | to=0 | width=14 | annotations-width=16 | period1=Hadean | period1-text= | period2=Archean | period2-text= | period3=Proterozoic | period3-text= | period3-nudge-up=0.5 | period4=Phanerozoic | period4-text= | period4-nudge-up=0.1 | period1-right=0.30 | period2-right=0.30 | period3-right=0.30 | period4-right=0.30 | period5 = Eoarchean | period5-text = | period6 = Paleoarchean | period6-text = | period7 = Mesoarchean | period7-text = | period8 = Neoarchean | period8-text = | period5-left=0.305 | period6-left=0.305 | period7-left=0.305 | period8-left=0.305 | period5-right=0.55 | period6-right=0.55 | period7-right=0.55 | period8-right=0.55 | period9 = Paleoproterozoic | period9-text = | period10 = Mesoproterozoic | period10-text = | period11 = Neoproterozoic | period11-text = | period12 = Paleozoic | period12-text = | period13 = Mesozoic | period13-text = | period14 = Cenozoic | period14-text = | period9-left=0.305 | period10-left=0.305 | period11-left=0.305 | period12-left=0.305 | period13-left=0.305 | period14-left=0.305 | period9-right=0.55 | period10-right=0.55 | period11-right=0.55 | period12-right=0.55 | period13-right=0.55 | period14-right=0.55 | period1-border-width=0.04 | period2-border-width=0.04 | period3-border-width=0.04 | period4-border-width=0.04 | period5-border-width=0.04 | period6-border-width=0.04 | period7-border-width=0.04 | period8-border-width=0.04 | period9-border-width=0.04 | period10-border-width=0.04 | period11-border-width=0.04 | period12-border-width=0.04 | period13-border-width=0.04 | period14-border-width=0.04 | period15-border-width=0.04 | period15 = | period15-right=-0.01 | period15-left=-0.08 | period15-colour=#393 | period15-text= | period15-border-colour=#c33 | bar1-colour=black | bar1-left = 0.30 | bar1-right = 0.305 | bar2-colour=black | bar2-left=0.551 | bar2-right = 0.554 | bar2-from=- |note1= and formed |note1-at=-4550 |note1-nudge-left=5.5 |note1-nudge-down=0.0 |note2= |note2-at=-4510 |note2-nudge-left=5.5 |note2-nudge-down=0.3 |note3=Cool surface, oceans, atmosphere |note3-at=-4450 |note3-nudge-left=5.5 |note3-nudge-up=0.0 |note4= |note4-at=-3900 |note4-nudge-left=5.5 |note4-nudge-down=0.0 | bar4-colour=gray | bar4-left=0.555 | bar4-right = 0.60 | bar4-from=-4000 | bar4-to=-3800 |note5= |note5-at=-4100 |note5-nudge-left=5.5 |note5-nudge-down=0.0 |note6= of atmosphere |note6-at=-2400 |note6-nudge-left=5.5 |note6-nudge-down=0.0 |note7=Earliest |note7-at=-0800 |note7-nudge-left=5.5 |note7-nudge-down=0.0 |note8= |note8-at=-0430 |note8-nudge-left=5.5 |note8-nudge-down=0.3 |note9= |note9-at=-580 |note9-nudge-left=5.5 |note9-nudge-down=0.7 |note10= |note10-at=-530 |note10-nudge-left=5.5 |note10-nudge-down=0.4 |note11=Earliest land s and s |note11-at=-470 |note11-nudge-left=5.5 |note11-nudge-down=0.1 |note12=Earliest land s |note12-at=-345 |note12-nudge-left=5.5 |note12-nudge-down=0.1 |note13=Earliest known |note13-at=-225 |note13-nudge-left=5.5 |note13-nudge-down=0.0 |note14=Extinction of non-avian dinosaurs |note14-at=-65 |note14-nudge-left=5.5 |note14-nudge-down=0.0 | caption=Scale: (Millions of years) }} The oldest fragments found on Earth are about 4.54 billion years old; this, coupled primarily with the dating of ancient deposits, has put the estimated age of Earth at around that time. The Moon has the same composition as Earth's but does not contain an -rich like the Earth's. Many scientists think that about 40 million years after the formation of Earth, it collided with , throwing into orbit crust material that formed the Moon. Another is that the Earth and Moon started to coalesce at the same time but the Earth, having much stronger than the early Moon, attracted almost all the iron particles in the area. Until 2001, the oldest rocks found on Earth were about 3.8 billion years old, leading scientists to estimate that the Earth's surface had been molten until then. Accordingly, they named this part of Earth's history the . However, analysis of s formed 4.4 Ga indicates that Earth's crust solidified about 100 million years after the planet's formation and that the planet quickly acquired oceans and an , which may have been capable of supporting life. Evidence from the Moon indicates that from 4 to 3.8 Ga it suffered a by debris that was left over from the formation of the , and the Earth should have experienced an even heavier bombardment due to its stronger gravity. While there is no direct evidence of conditions on Earth 4 to 3.8 Ga, there is no reason to think that the Earth was not also affected by this late heavy bombardment. This event may well have stripped away any previous atmosphere and oceans; in this case es and water from impacts may have contributed to their replacement, although from es on Earth would have supplied at least half. However, if subsurface microbial life had evolved by this point, it would have survived the bombardment. Earliest evidence for life on Earth The earliest identified organisms were minute and relatively featureless, and their fossils look like small rods that are very difficult to tell apart from structures that arise through abiotic physical processes. The oldest undisputed evidence of life on Earth, interpreted as fossilized bacteria, dates to 3 Ga. Other finds in rocks dated to about 3.5 Ga have been interpreted as bacteria, with evidence also seeming to show the presence of life 3.8 Ga. However, these analyses were closely scrutinized, and non-biological processes were found which could produce all of the "signatures of life" that had been reported. While this does not prove that the structures found had a non-biological origin, they cannot be taken as clear evidence for the presence of life. Geochemical signatures from rocks deposited 3.4 Ga have been interpreted as evidence for life, although these statements have not been thoroughly examined by critics. Evidence for fossilized microorganisms considered to be 3,770 million to 4,280 million years old was found in the Nuvvuagittuq Greenstone Belt in Quebec, Canada, although the evidence is disputed as inconclusive. Environmental and evolutionary impact of microbial mats s in , }} Microbial mats are multi-layered, multi-species colonies of bacteria and other organisms that are generally only a few millimeters thick, but still contain a wide range of chemical environments, each of which favors a different set of microorganisms. To some extent each mat forms its own , as the by-products of each group of microorganisms generally serve as "food" for adjacent groups. s are stubby pillars built as microorganisms in mats slowly migrate upwards to avoid being smothered by sediment deposited on them by water. There has been vigorous debate about the validity of alleged fossils from before 3 Ga, with critics arguing that so-called stromatolites could have been formed by non-biological processes. In 2006, another find of stromatolites was reported from the same part of Australia as previous ones, in rocks dated to 3.5 Ga. In modern underwater mats the top layer often consists of photosynthesizing which create an oxygen-rich environment, while the bottom layer is oxygen-free and often dominated by hydrogen sulfide emitted by the organisms living there. It is estimated that the appearance of by bacteria in mats increased biological productivity by a factor of between 100 and 1,000. The used by oxygenic photosynthesis is water, which is much more plentiful than the geologically produced reducing agents required by the earlier non-oxygenic photosynthesis. From this point onwards life itself produced significantly more of the resources it needed than did geochemical processes. Oxygen is toxic to organisms that are not adapted to it, but greatly increases the efficiency of oxygen-adapted organisms. Oxygen became a significant component of Earth's atmosphere about 2.4 Ga. Although eukaryotes may have been present much earlier, the oxygenation of the atmosphere was a prerequisite for the evolution of the most complex eukaryotic cells, from which all multicellular organisms are built. The boundary between oxygen-rich and oxygen-free layers in microbial mats would have moved upwards when photosynthesis shut down overnight, and then downwards as it resumed on the next day. This would have created for organisms in this intermediate zone to acquire the ability to tolerate and then to use oxygen, possibly via , where one organism lives inside another and both of them benefit from their association. Cyanobacteria have the most complete biochemical "toolkits" of all the mat-forming organisms. Hence they are the most self-sufficient of the mat organisms and were well-adapted to strike out on their own both as floating mats and as the first of the , providing the basis of most marine food chains. Diversification of eukaryotes | 1= | 1= ( s, , , and s) | label2= | 2= | label3= | 3= ( , , and ) }} | label2= | 2= | label3= | 3= | label2 = | 2= | label3= | 3= ( s) | label2= | 2= }} }} }} }} One possible family tree of eukaryotes Chromatin, nucleus, endomembrane system, and mitochondria Eukaryotes may have been present long before the oxygenation of the atmosphere, but most modern eukaryotes require oxygen, which their use to fuel the production of , the internal energy supply of all known cells. In the 1970s it was proposed and, after much debate, widely accepted that eukaryotes emerged as a result of a sequence of endosymbiosis between " s." For example: a y microorganism invaded a large prokaryote, probably an n, but the attack was neutralized, and the attacker took up residence and evolved into the first of the mitochondria; one of these s later tried to swallow a photosynthesizing cyanobacterium, but the victim survived inside the attacker and the new combination became the ancestor of s; and so on. After each endosymbiosis began, the partners would have eliminated unproductive duplication of genetic functions by re-arranging their genomes, a process which sometimes involved transfer of genes between them. Another hypothesis proposes that mitochondria were originally - or -metabolising endosymbionts, and became oxygen-consumers later. On the other hand, mitochondria might have been part of eukaryotes' original equipment. There is a debate about when eukaryotes first appeared: the presence of s in Australian s may indicate that eukaryotes were present 2.7 Ga; however, an analysis in 2008 concluded that these chemicals infiltrated the rocks less than 2.2 Ga and prove nothing about the origins of eukaryotes. Fossils of the have been reported in 1.85 billion-year-old rocks (originally dated to 2.1 Ga but later revised), and indicates that eukaryotes with organelles had already evolved. A diverse collection of fossil algae were found in rocks dated between 1.5 and 1.4 Ga. The earliest known fossils of date from 1.43 Ga. Plastids s, the superclass of s of which s are the best-known exemplar, are thought to have originated from cyanobacteria. The symbiosis evolved around 1.5 Ga and enabled eukaryotes to carry out . Three evolutionary lineages have since emerged in which the plastids are named differently: chloroplasts in and plants, s in and s in the s. Sexual reproduction and multicellular organisms Evolution of sexual reproduction The defining characteristics of in eukaryotes are and . There is much in this kind of reproduction, in which offspring receive 50% of their genes from each parent, in contrast with , in which there is no recombination. Bacteria also exchange DNA by , the benefits of which include resistance to s and other s, and the ability to utilize new s. However, conjugation is not a means of reproduction, and is not limited to members of the same species – there are cases where bacteria transfer DNA to plants and animals. On the other hand, bacterial transformation is clearly an adaptation for transfer of DNA between bacteria of the same species. Bacterial transformation is a complex process involving the products of numerous bacterial genes and can be regarded as a bacterial form of sex. This process occurs naturally in at least 67 prokaryotic species (in seven different phyla). Sexual reproduction in eukaryotes may have evolved from bacterial transformation. (Also see .) The disadvantages of sexual reproduction are well-known: the genetic reshuffle of recombination may break up favorable combinations of genes; and since males do not directly increase the number of offspring in the next generation, an asexual population can out-breed and displace in as little as 50 generations a sexual population that is equal in every other respect. Nevertheless, the great majority of animals, plants, fungi and s reproduce sexually. There is strong evidence that sexual reproduction arose early in the history of eukaryotes and that the genes controlling it have changed very little since then. How sexual reproduction evolved and survived is an unsolved puzzle. '' may have been an early , or a n. It apparently re-arranged itself into fewer but larger main masses as the sediment grew deeper round its base.}} The suggests that sexual reproduction provides protection against s, because it is easier for parasites to evolve means of overcoming the defenses of genetically identical s than those of sexual species that present moving targets, and there is some experimental evidence for this. However, there is still doubt about whether it would explain the survival of sexual species if multiple similar clone species were present, as one of the clones may survive the attacks of parasites for long enough to out-breed the sexual species. Furthermore, contrary to the expectations of the Red Queen hypothesis, Kathryn A. Hanley et al. found that the prevalence, abundance and mean intensity of mites was significantly higher in sexual geckos than in asexuals sharing the same habitat. In addition, biologist Matthew Parker, after reviewing numerous genetic studies on plant disease resistance, failed to find a single example consistent with the concept that pathogens are the primary selective agent responsible for sexual reproduction in the host. 's deterministic mutation hypothesis (DMH) assumes that each organism has more than one harmful mutation and the combined effects of these mutations are more harmful than the sum of the harm done by each individual mutation. If so, sexual recombination of genes will reduce the harm that bad mutations do to offspring and at the same time eliminate some bad mutations from the by isolating them in individuals that perish quickly because they have an above-average number of bad mutations. However, the evidence suggests that the DMH's assumptions are shaky because many species have on average less than one harmful mutation per individual and no species that has been investigated shows evidence of between harmful mutations. (Further criticisms of this hypothesis are discussed in the article ) The random nature of recombination causes the relative abundance of alternative traits to vary from one generation to another. This is insufficient on its own to make sexual reproduction advantageous, but a combination of genetic drift and natural selection may be sufficient. When chance produces combinations of good traits, natural selection gives a large advantage to lineages in which these traits become genetically linked. On the other hand, the benefits of good traits are neutralized if they appear along with bad traits. Sexual recombination gives good traits the opportunities to become linked with other good traits, and mathematical models suggest this may be more than enough to offset the disadvantages of sexual reproduction. Other combinations of hypotheses that are inadequate on their own are also being examined. The adaptive function of sex today remains a major unresolved issue in biology. The competing models to explain the adaptive function of sex were reviewed by John A. Birdsell and . The hypotheses discussed above all depend on the possible beneficial effects of random genetic variation produced by genetic recombination. An alternative view is that sex arose and is maintained, as a process for repairing DNA damage, and that the genetic variation produced is an occasionally beneficial byproduct. Multicellularity The simplest definitions of "multicellular," for example "having multiple cells," could include cyanobacteria like . Even a technical definition such as "having the same genome but different types of cell" would still include some of the green algae , which have cells that specialize in reproduction. Multicellularity evolved independently in organisms as diverse as s and other animals, fungi, plants, , cyanobacteria, s and . For the sake of brevity, this article focuses on the organisms that show the greatest specialization of cells and variety of cell types, although this approach to the could be regarded as "rather ." solves a maze. The mold (yellow) explored and filled the maze (left). When the researchers placed sugar (red) at two separate points, the mold concentrated most of its mass there and left only the most efficient connection between the two points (right).}} The initial advantages of multicellularity may have included: more efficient sharing of nutrients that are digested outside the cell, increased resistance to predators, many of which attacked by engulfing; the ability to resist currents by attaching to a firm surface; the ability to reach upwards to filter-feed or to obtain sunlight for photosynthesis; the ability to create an internal environment that gives protection against the external one; and even the opportunity for a group of cells to behave "intelligently" by sharing information. These features would also have provided opportunities for other organisms to diversify, by creating more varied environments than flat microbial mats could. Multicellularity with differentiated cells is beneficial to the organism as a whole but disadvantageous from the point of view of individual cells, most of which lose the opportunity to reproduce themselves. In an asexual multicellular organism, rogue cells which retain the ability to reproduce may take over and reduce the organism to a mass of undifferentiated cells. Sexual reproduction eliminates such rogue cells from the next generation and therefore appears to be a prerequisite for complex multicellularity. The available evidence indicates that eukaryotes evolved much earlier but remained inconspicuous until a rapid diversification around 1 Ga. The only respect in which eukaryotes clearly surpass bacteria and archaea is their capacity for variety of forms, and sexual reproduction enabled eukaryotes to exploit that advantage by producing organisms with multiple cells that differed in form and function. By comparing the composition of transcription factor families and regulatory network motifs between unicellular organisms and multicellular organisms, scientists found there are many novel transcription factor families and three novel types of regulatory network motifs in multicellular organisms, and novel family transcription factors are preferentially wired into these novel network motifs which are essential for multicullular development. These results propose a plausible mechanism for the contribution of novel-family transcription factors and novel network motifs to the origin of multicellular organisms at transcriptional regulatory level. Fossil evidence The fossils, dated to 2.1 Ga, are the earliest known fossil organisms that are clearly multicellular. They may have had differentiated cells. Another early multicellular fossil, , dated to 1.7 Ga, appears to consist of virtually identical cells. The red algae called , dated at 1.2 Ga, is the earliest known organism that certainly has differentiated, specialized cells, and is also the oldest known sexually reproducing organism. The 1.43 billion-year-old fossils interpreted as fungi appear to have been multicellular with differentiated cells. The "string of beads" organism , found in rocks dated from 1.5 Ga to 900 Ma, may have been an early metazoan; however, it has also been interpreted as a colonial n. Emergence of animals ns |1= s ( s, s, s) |label2= s |2= ( s, s, s, etc.) |label2= |2= ( s, s, s, etc.) }} }} |label2= |2= }} |2= ( , s, s) |3= ( ) }} |2= |3= (sponges): }} |2= : a & }} |2= |3= }} }} A family tree of the animals Animals are multicellular eukaryotes, and are distinguished from plants, algae, and fungi by lacking s. All animals are , if only at certain life stages. All animals except sponges have bodies differentiated into separate s, including s, which move parts of the animal by contracting, and , which transmits and processes signals. The earliest widely accepted animal fossils are the rather modern-looking ns (the group that includes , s and ), possibly from around , although fossils from the can only be dated approximately. Their presence implies that the cnidarian and n lineages had already diverged. The Ediacara biota, which flourished for the last 40 million years before the start of the , were the first animals more than a very few centimetres long. Many were flat and had a "quilted" appearance, and seemed so strange that there was a proposal to classify them as a separate , . Others, however, have been interpreted as early s ( ), s ( ), and s ( , ). There is still debate about the classification of these specimens, mainly because the diagnostic features which allow taxonomists to classify more recent organisms, such as similarities to living organisms, are generally absent in the Ediacarans. However, there seems little doubt that Kimberella was at least a bilaterian animal, in other words, an animal significantly more complex than the cnidarians. The are a very mixed collection of fossils found between the Late Ediacaran and periods. The earliest, , shows signs of successful defense against predation and may indicate the start of an . Some tiny Early Cambrian shells almost certainly belonged to molluscs, while the owners of some "armor plates," and , were eventually identified when more complete specimens were found in Cambrian n that preserved soft-bodied animals. '' made the largest single contribution to modern interest in the Cambrian explosion.}} In the 1970s there was already a debate about whether the emergence of the modern phyla was "explosive" or gradual but hidden by the shortage of animal fossils. A re-analysis of fossils from the lagerstätte increased interest in the issue when it revealed animals, such as , which did not fit into any known . At the time these were interpreted as evidence that the modern phyla had evolved very rapidly in the Cambrian explosion and that the Burgess Shale's "weird wonders" showed that the Early Cambrian was a uniquely experimental period of animal evolution. Later discoveries of similar animals and the development of new theoretical approaches led to the conclusion that many of the "weird wonders" were evolutionary "aunts" or "cousins" of modern groups—for example that Opabinia was a member of the s, a group which includes the ancestors of the arthropods, and that it may have been closely related to the modern s. Nevertheless, there is still much debate about whether the Cambrian explosion was really explosive and, if so, how and why it happened and why it appears unique in the history of animals. Deuterostomes and the first vertebrates s were among the earliest vertebrates with s. }} Most of the animals at the heart of the Cambrian explosion debate are s, one of the two main groups of complex animals. The other major group, the s, contains s such as and s (echinoderms), as well as s (see below). Many echinoderms have hard "shells," which are fairly common from the Early Cambrian small shelly fauna onwards. Other deuterostome groups are soft-bodied, and most of the significant Cambrian deuterostome fossils come from the , a lagerstätte in . The chordates are another major deuterostome group: animals with a distinct dorsal nerve cord. Chordates include soft-bodied invertebrates such as s as well as vertebrates—animals with a backbone. While tunicate fossils predate the Cambrian explosion, the Chengjiang fossils and appear to be true vertebrates, and Haikouichthys had distinct , which may have been slightly . Vertebrates with s, such as the s, first appeared in the Late . Colonization of land Adaptation to life on land is a major challenge: all land organisms need to avoid drying-out and all those above microscopic size must create special structures to withstand gravity; and systems have to change; reproductive systems cannot depend on water to carry s and towards each other. Although the earliest good evidence of land plants and animals dates back to the Ordovician period ( ), and a number of microorganism lineages made it onto land much earlier, modern land s only appeared in the Late , about . In May 2017, evidence of the may have been found in 3.48-billion-year-old and other related mineral deposits (often found around s and s) uncovered in the of . In July 2018, scientists reported that the earliest life on land may have been living on land 3.22 billion years ago. In May 2019, scientists reported the discovery of a ized , named , in the , that may have grown on land a billion years ago, well before s were living on land. Evolution of terrestrial antioxidants Oxygen is a potent whose accumulation in terrestrial atmosphere resulted from the development of over 3 Ga, in cyanobacteria (blue-green algae), which were the most primitive oxygenic photosynthetic organisms. Brown algae accumulate inorganic mineral s such as , , , iron, , , and which is concentrated more than 30,000 times the concentration of this element in seawater. Protective endogenous antioxidant enzymes and exogenous dietary antioxidants helped to prevent oxidative damage. Most marine mineral antioxidants act in the cells as essential s in and antioxidant . When plants and animals began to enter rivers and land about 500 Ma, environmental deficiency of these marine mineral antioxidants was a challenge to the evolution of terrestrial life. Terrestrial plants slowly optimized the production of “new” endogenous antioxidants such as , s, s, s, etc. A few of these appeared more recently, in last 200–50 Ma, in s and s of plants. In fact, angiosperms (the dominant type of plant today) and most of their antioxidant pigments evolved during the period. Plants employ antioxidants to defend their structures against produced during photosynthesis. Animals are exposed to the same oxidants, and they have evolved endogenous enzymatic antioxidant systems. is the most primitive and abundant electron-rich essential element in the diet of marine and terrestrial organisms, and as iodide acts as an and has this ancestral antioxidant function in all iodide-concentrating cells from primitive marine algae to more recent terrestrial vertebrates. Evolution of soil Before the colonization of land, , a combination of mineral particles and decomposed , did not exist. Land surfaces would have been either bare rock or unstable sand produced by . Water and any nutrients in it would have drained away very quickly. In the in Sweden for example maximum depth of itization by is about 5 m, in contrast nearby kaolin deposits developed in the are . It has been argued that in the late Neoproterozoic was a dominant process of erosion of surface material due to the on land. s growing on }} Films of cyanobacteria, which are not plants but use the same photosynthesis mechanisms, have been found in modern deserts, and only in areas that are unsuitable for s. This suggests that microbial mats may have been the first organisms to colonize dry land, possibly in the Precambrian. Mat-forming cyanobacteria could have gradually evolved resistance to desiccation as they spread from the seas to s and then to land. s, which are combinations of a fungus (almost always an ) and one or more photosynthesizers (green algae or cyanobacteria), are also important colonizers of lifeless environments, and their ability to break down rocks contributes to in situations where plants cannot survive. The earliest known ascomycete fossils date from in the . Soil formation would have been very slow until the appearance of burrowing animals, which mix the mineral and organic components of soil and whose are a major source of the organic components. Burrows have been found in Ordovician sediments, and are attributed to s ("worms") or arthropods. Plants and the Late Devonian wood crisis , a from the }} s from the Middle }} In aquatic algae, almost all cells are capable of photosynthesis and are nearly independent. Life on land required plants to become internally more complex and specialized: photosynthesis was most efficient at the top; roots were required in order to extract water from the ground; the parts in between became supports and transport systems for water and nutrients. Spores of land plants, possibly rather like , have been found in Middle Ordovician rocks dated to about . In rocks , there are fossils of actual plants including es such as ; most were under high, and some appear closely related to s, the group that includes s. By the Late Devonian , trees such as were so abundant that they changed river systems from mostly to mostly ing, because their roots bound the soil firmly. In fact, they caused the "Late Devonian wood crisis" because: * They removed more carbon dioxide from the atmosphere, reducing the and thus causing an in the period. In later ecosystems the carbon dioxide "locked up" in wood is returned to the atmosphere by decomposition of dead wood. However, the earliest fossil evidence of fungi that can decompose wood also comes from the Late Devonian. * The increasing depth of plants' roots led to more washing of nutrients into rivers and seas by rain. This caused s whose high consumption of oxygen caused s in deeper waters, increasing the extinction rate among deep-water animals. Land invertebrates Animals had to change their feeding and systems, and most land animals developed of their eggs. The difference in between water and air required changes in their eyes. On the other hand, in some ways movement and breathing became easier, and the better transmission of high-frequency sounds in air encouraged the development of . contributed to the total by each of animals. is the phylum with the most individual organisms while has the most species.}} The oldest known air-breathing animal is , an n from the Middle Silurian, about . Its air-breathing, terrestrial nature is evidenced by the presence of s, the openings to . However, some earlier s from the Cambrian-Ordovician boundary about are interpreted as the tracks of large arthropods on coastal s, and may have been made by s, which are thought to be evolutionary "aunts" of s. Other trace fossils from the Late Ordovician a little over probably represent land invertebrates, and there is clear evidence of numerous arthropods on coasts and s shortly before the Silurian-Devonian boundary, about , including signs that some arthropods ate plants. Arthropods were well to colonise land, because their existing jointed exoskeletons provided protection against desiccation, support against gravity and a means of locomotion that was not dependent on water. The of other major invertebrate groups on land is poor: none at all for non- s, s or ns; some parasitic nematodes have been fossilized in ; annelid worm fossils are known from the Carboniferous, but they may still have been aquatic animals; the earliest fossils of s on land date from the Late Carboniferous, and this group may have had to wait until became abundant enough to provide the moist conditions they need. The earliest confirmed fossils of flying s date from the Late Carboniferous, but it is thought that insects developed the ability to fly in the Early Carboniferous or even Late Devonian. This gave them a wider range of s for feeding and breeding, and a means of escape from predators and from unfavorable changes in the environment. About 99% of modern insect species fly or are descendants of flying species. Early land vertebrates '' changed views about the early evolution of s.}} |2= e | 2= | 2= '' | 2= '' | 2= '' |2= s |2= |2= s }} }} }} }} }} }} }} }} }} Family tree of s s, vertebrates with four limbs, evolved from other n fish over a relatively short timespan during the Late Devonian ( ). The early groups are grouped together as . They retained aquatic, fry-like s, a system still seen in . Iodine and T4/T3 stimulate the amphibian metamorphosis and the transforming the aquatic, vegetarian tadpole into a “more evoluted” terrestrial, carnivorous frog with better neurological, visuospatial, olfactory and cognitive abilities for hunting. The new hormonal action of T3 was made possible by the formation of T3-receptors in the cells of vertebrates. Firstly, about 600-500 million years ago, in primitive Chordata appeared the alpha T3-receptors with a metamorphosing action and then, about 250-150 million years ago, in the Birds and Mammalia appeared the beta T3-receptors with metabolic and thermogenetic actions. From the 1950s to the early 1980s it was thought that tetrapods evolved from fish that had already acquired the ability to crawl on land, possibly in order to go from a pool that was drying out to one that was deeper. However, in 1987, nearly complete fossils of from about showed that this Late Devonian animal had s and both s and s, but could never have survived on land: its limbs and its wrist and ankle joints were too weak to bear its weight; its ribs were too short to prevent its lungs from being squeezed flat by its weight; its fish-like tail fin would have been damaged by dragging on the ground. The current hypothesis is that Acanthostega, which was about long, was a wholly aquatic predator that hunted in shallow water. Its skeleton differed from that of most fish, in ways that enabled it to raise its head to breathe air while its body remained submerged, including: its jaws show modifications that would have enabled it to gulp air; the bones at the back of its skull are locked together, providing strong attachment points for muscles that raised its head; the head is not joined to the and it has a distinct neck. The Devonian proliferation of land plants may help to explain why air breathing would have been an advantage: leaves falling into streams and rivers would have encouraged the growth of aquatic vegetation; this would have attracted grazing invertebrates and small fish that preyed on them; they would have been attractive prey but the environment was unsuitable for the big marine predatory fish; air-breathing would have been necessary because these waters would have been short of oxygen, since warm water holds less than cooler marine water and since the decomposition of vegetation would have used some of the oxygen. Later discoveries revealed earlier transitional forms between Acanthostega and completely fish-like animals. Unfortunately, there is then a gap ( ) of about 30 Ma between the fossils of ancestral tetrapods and Middle Carboniferous fossils of vertebrates that look well-adapted for life on land. Some of these look like early relatives of modern amphibians, most of which need to keep their skins moist and to lay their eggs in water, while others are accepted as early relatives of the s, whose waterproof skin and egg membranes enable them to live and breed far from water. Dinosaurs, birds and mammals s |1= s |1= s |2= s |2= |2= s }} }} }} }} |label2= s |2= s; whether s belong here is debated |label2= |2= and (extinct) |label2= s |2= (extinct) |label2= |2= ( s and s) |label2= s |2= ns |label3= |3= s (extinct) |label2= s |2= ns |1= s |1= s }} |2= s (extinct) }} |label2= |2= ns (extinct) }} }} }} }} }} }} }} }} }} }} Possible family tree of s, s and s Amniotes, whose eggs can survive in dry environments, probably evolved in the Late Carboniferous period ( ). The earliest fossils of the two surviving amniote groups, s and s, date from around . The synapsid s and their descendants the s are the most common land vertebrates in the best-known Permian ( ) fossil beds. However, at the time these were all in zones at middle s, and there is evidence that hotter, drier environments nearer the Equator were dominated by sauropsids and amphibians. The wiped out almost all land vertebrates, as well as the great majority of other life. During the slow recovery from this catastrophe, estimated to have taken 30 million years, a previously obscure sauropsid group became the most abundant and diverse terrestrial vertebrates: a few fossils of ("ruling lizard forms") have been found in Late Permian rocks, but, by the , archosaurs were the dominant land vertebrates. Dinosaurs distinguished themselves from other archosaurs in the Late Triassic, and became the dominant land vertebrates of the Jurassic and Cretaceous periods ( ). During the Late Jurassic, birds evolved from small, predatory dinosaurs. The first birds inherited teeth and long, bony tails from their dinosaur ancestors, but some had developed horny, toothless s by the very Late Jurassic and short tails by the . While the archosaurs and dinosaurs were becoming more dominant in the Triassic, the successors of the therapsids evolved into small, mainly nocturnal s. This ecological role may have promoted the , for example nocturnal life may have accelerated the development of y ("warm-bloodedness") and hair or fur. By in the there were animals that were very like today's mammals in a number of respects. Unfortunately, there is a gap in the fossil record throughout the Middle Jurassic. However, fossil teeth discovered in indicate that the split between the lineage leading to s and the one leading to other living mammals had occurred by . After dominating land vertebrate niches for about 150 Ma, the non-avian dinosaurs perished in the Cretaceous–Paleogene extinction event ( ) along with many other groups of organisms. Mammals throughout the time of the dinosaurs had been restricted to a narrow range of , sizes and shapes, but increased rapidly in size and diversity after the extinction, with s taking to the air within 13 million years, and ns to the sea within 15 million years. Flowering plants The first flowering plants appeared around 130 Ma. The 250,000 to 400,000 species of flowering plants outnumber all other ground plants combined, and are the dominant vegetation in most terrestrial ecosystems. There is fossil evidence that flowering plants diversified rapidly in the Early Cretaceous, from , and that their rise was associated with that of insects. Among modern flowering plants are thought to be close to the common ancestor of the group. However, paleontologists have not succeeded in identifying the earliest stages in the evolution of flowering plants. Social insects mounds have survived a bush fire.}} The social insects are remarkable because the great majority of individuals in each colony are sterile. This appears contrary to basic concepts of evolution such as natural selection and the . In fact, there are very few insect species: only 15 out of approximately 2,600 living of insects contain eusocial species, and it seems that eusociality has evolved independently only 12 times among arthropods, although some eusocial lineages have diversified into several families. Nevertheless, social insects have been spectacularly successful; for example although s and s account for only about 2% of known insect species, they form over 50% of the total mass of insects. Their ability to control a territory appears to be the foundation of their success. The sacrifice of breeding opportunities by most individuals has long been explained as a consequence of these species' unusual method of , which has the paradoxical consequence that two sterile worker daughters of the same queen share more genes with each other than they would with their offspring if they could breed. However, and argue that this explanation is faulty: for example, it is based on , but there is no evidence of in colonies that have multiple queens. Instead, they write, eusociality evolves only in species that are under strong pressure from predators and competitors, but in environments where it is possible to build "fortresses"; after colonies have established this security, they gain other advantages through co-operative . In support of this explanation they cite the appearance of eusociality in mole rats, which are not haplodiploid. The earliest fossils of insects have been found in Early Devonian rocks from about , which preserve only a few varieties of flightless insect. The from the Late Carboniferous, about , include about 200 species, some gigantic by modern standards, and indicate that insects had occupied their main modern ecological niches as s, s and insectivores. Social termites and ants first appear in the Early Cretaceous, and advanced social bees have been found in Late Cretaceous rocks but did not become abundant until the Middle . Humans The idea that, along with other life forms, modern-day humans evolved from an ancient, common ancestor was proposed by in 1844 and taken up by in 1871. Modern humans evolved from a lineage of upright-walking s that has been traced back over to . The first known s were made about , apparently by , and were found near animal bones that bear scratches made by these tools. The earliest s had -sized brains, but there has been a fourfold increase in the last 3 Ma; a statistical analysis suggests that hominine brain sizes depend almost completely on the date of the fossils, while the species to which they are assigned has only slight influence. There is a long-running debate about whether modern humans evolved simultaneously from existing advanced hominines or are descendants of a , which then migrated all over the world less than 200,000 years ago and replaced previous hominine species. There is also debate about whether anatomically modern humans had an intellectual, cultural and technological under 100,000 years ago and, if so, whether this was due to neurological changes that are not visible in fossils. Footnotes References Category:History of the world